1. Field of the Invention
The present invention relates to a coil spring device adapted for use in a car suspension system having at least two axes or as shock absorbing means for various apparatus.
2. Description of the Related Art
Coil springs, such as compression springs and tension springs, are used in various applications. If a load is applied to a conventional coil spring, the degree of freedom of the spring may be supposed to be based on six factors, including linear displacements along the X-, Y-, and Z-axes and rotational displacements .PSI..sub.X, .PSI..sub.Y and .PSI..sub.Z around the X-, Y-, and Z-axes, as shown in FIG. 20. Axial force Ps along axis S of wire 1, torsional moment M.sub.S, shearing force P.eta., bending moment M.eta., etc. are produced in various parts of wire 1 of the spring. Accordingly, stresses produced by the combination of these forces are not uniformly distributed on the surface of wire 1, with respect to the direction of arrow F of FIG. 20 along the wire.
In prior art coil spring device 2 shown in FIG. 21, for example, end turns 4 and 5 of coil spring 3 are bound by means of spring seats 6 and 7 so that a vertical load is applied to spring 3 via seats 6 and 7. If upper spring seat 6 is pressed toward lower spring seat 7, for example, coil spring 3 is brought to the so-called parallel compressed state. Since end turns 4 and 5 are fixed and prevented from rotating by spring seats 6 and 7, moreover, spring 3 is allowed only to move in the direction of the Z-axis of FIG. 20. An examination of the stress in the direction along wire 1 of coil spring 3 in the parallel compressed state revealed a wavy stress distribution, as shown in FIG. 22. By way of example, conventional coil spring 3 may enjoy the wire diameter of 9.5 mm, number of turns of 4.0, pitch diameter of 80 mm, free height of 320 mm, and applied load of 160 kgf. FIG. 22 shows the maximum inside principal stress and the maximum inside shearing stress. It is known that the maximum outside principal stress and the maximum outside shearing stress behave in like manner.
If the waviness of the stress distribution so remarkable, as in the case of conventional coil spring device 2, an extremely high stress is locally applied to some regions of the surface of wire 1. If these surface regions have any surface defects, such as scratches, the wire is liable to be broken.
Published Unexamined Japanese Patent Publication No 60-85003 (Prior Art Example 1) is proposed as a conventional example of an improvement of an end support structure for a coil spring wire. In this example, one end of the spring wire is supported by means of a support element, such as a steel sphere, for rotation in all directions, whereby a bending stress produced at the wire end is reduced. In a suspension spring disclosed in Published Unexamined Japanese Patent Publication No. 56-60707 (Prior Art Example 2), moreover, spring seats for retaining a coil spring are each rockably supported by means of a pin or a steel sphere, whereby a bending moment on the spring is canceled. In Prior Art Examples 1 and 2, the stress distribution in the direction along wire 1 is not taken into consideration at all.
If the wire end is supported for rotation in all directions, as in Prior Art Example 1, the construction of wire end support means is complicated, and the support is liable to be unstable. In Example 1, although the spring can support a load in the direction of compression, the wire end cannot be bound for a load in the tensile direction.
In Prior Art Example 2, on the other hand, the spring requires end turns and spring seats. When the wire is compressed, therefore, the end turns and the seats are brought into contact with one another, thereby causing tappings, which entail noises. Since the spring seats and the end turns are heavy in weight, the whole coil spring device is inevitably heavy.